In general, the ultra-sound imaging technique uses the pulse-echo method. According to such method, pulses of ultrasonic energy are generated by a piezoelectric transducer, for example, made of lead zirconate-titanate ceramic. Each short pulse is focused to a narrow beam. This focused beam is transmitted through a suitable conducting medium, e.g., water, into the body of a patient. A portion of the ultrasonic energy is reflected back toward the transducer at interfaces between various different structures of the body due to mechanical impedance discontinuities at the interface. The transducer converts the reflected mechanical energy into electrical signals. The time of arrival of the returning reflected signals indicates the position within the body of the interface. In other words, the timed spacing between the reflected signals or echoes is proportional to the physical spacing of the respective reflecting interfaces within the body. The amplitude of the echo is a function of the characteristics of the structures forming the interface.
The image-representing electrical signals corresponding to the characteristics of the reflected mechanical energy are then displayed on a display device. One type of display is termed an "A-scan" continuous display on which the electrical signals representing the reflected echo pulses are applied to the vertical deflection plates of a conventional cathode ray tube. The output of a time base or sweep generator is applied to the horizontal deflection plates of the CRT. The pulse-echo process is continuously repeated in synchronism with the sweep to produce a continuous display. In the display, time is proportional to range, and the height of the vertical deflections is a function of the reflected echo strength.
Another type of display device commonly employed is termed a "B-scan" display. Such a display is comparable to a conventional television display. In such a system, the reflected echo signals modulate the brightness of the display at each point scanned. Strongly reflecting internal structures, such as hardened artery walls, appear brighter on the display than weakly reflecting structures. This grey scale produces a useful diagnostic tool. A plurality of scan lines can be produced by scanning the ultrasonic beam produced by the transducer, either by a mechanical sector scan or a phased-array radar sector scan, at a predetermined rate and in a predetermined direction across the surface of the patient. The plurality of scan lines so produced can be used to yield a display of a cross-sectional picture in the plane of the scan produced by the reflector-scanner, which scans mechanically over a desired angle.
A limitation on diagnostic methodologies based on cardiac ultrasound imaging or "echocardiology" arises from the inability of conventional systems to image very rapidly moving bodily structures. Examples of such structure are: the aortic valve opening, which has a duration of approximately 30 milliseconds in an adult human and less in children, vibrations in various cardiac structures arising from regurgitant flow, and pediatric heart motion.
In systems which use the above-described "A-scan" continuous display, in which the images are displayed on a CRT, the inability to visually perceive extremely fast events results from limitations of the human observer, whose eye cannot perceive such rapid changes in real time. Thus, although the CRT is capable of displaying images of fast moving structures, the technique fails because the human observer cannot keep up with image update rates beyond a given threshold.
In systems which use the above-described "B-scan" display, where the images are presented on a standard CRT television-type display, the effective frame rate or image update rate is limited by the conventional television frame rate. According to the American television standard, this rate is 30 frames per second; and, according to the European standard, it is 25 frames per second. Even if the reflected echo signals corresponding to rapidly moving structures are recorded on a videotape, playing the tape in slow motion will not provide a system capable of displaying images corresponding to a "real time" rate faster than the above-noted conventional rates because the videotape is limited in that it is adapted for use with devices which have an acquisition rate equal to that of a standard television camera. In other words, the real time acquisition rate is 30 frames per second, and the display merely reflects this acquisition rate, but in slow motion. Thus, in such systems, real time events occuring at a rate faster than 30 events per second are beyond the capacity of the system.
Due to the these considerations, conventional ultra-sound systems limit their image acquisition modes such that they do not exceed 30 frames per second. Systems which exceed this acquisition rate are either limited by the television frame rate or by displays where higher frame rates cannot be appreciated.
It is, therefore, an object of the present invention to provide a new and improved method and apparatus for high-speed ultrasonic imaging, in which the above-described barriers to high-speed acquisition in the prior art systems are overcome.